1. Field of the Invention
This invention relates to a novel method for preparing extremely small amorphous magnetic cobalt-phosphorus (Co-P) alloy particles having selectively controlled coercivity by chemical reduction from solutions including controlled palladium cation (Pd.sup.+.sup.+) catalyst concentrations. Such particles are suitable for use in magnetic recording media, permanent magnets, magnetic cores, and in magnetically responsive fluid suspensions, such as magnetic or electrostrictive clutch couplings or the like.
2. Description of the Prior Art
In the prior art, magnetic alloys have been prepared in numerous ways. In one common type of preparation, solutions of cobalt, iron, or nickel salts, or mixtures thereof, are subjected to chemical reduction by the action of a reducing agent on the metal cations. In the prior art, such chemical or electroless reduction procedures have most often been carried out to produce continuous films or coatings. The pioneering effort with regard to electroless cobalt plating is detailed in U.S. Pat. No. 2,532,284. In such electroless plating procedures, reducing agents have commonly been of the hypophosphite, boron-nitrogen, borohydride, or organic formate type. It had been observed that in such electroless film plating procedures the plating bath is sometimes subjected to spontaneous decomposition, whereby a large portion of the metal cation content of the solution is vigorously and quickly reduced to a metallic state. The metallic material thus produced is normally a mixture of particles and discontinuous film covering a wide range of sizes, shapes and coercivities. Such catastrophic decomposition during film plating is usually brought about by excessive heating of the electroless solution, a change in the pH of the solution, the build-up of nucleating sites of the bath or the addition of catalytic material to the bath.
Recently, electroless-type baths including cobalt, iron or nickel salts have been used to intentionally produce finely divided particles having uniform size and useful magnetic characteristics. In such controlled decomposition reactions temperature, pH, and metal and reducing agent concentration parameters have been utilized to vary the physical properties, primarily the size, of the particles. To the extent that the magnetic properties of the particles are a function of size, the magnetic properties are also affected by these parameters. The catalytic material most often used for initiating controlled chemical reduction of magnetic metal salts to form particles has been finely divided palladium metal or salts of palladium in the form of a solution including hydrochloric acid. Recently, in U.S. Pat. No. 3,494,760 production of uniform magnetic Co-P particles was reported as having been accomplished by halting the initial palladium catalyzed reaction, removing the catalytic reaction particles, and then utilizing the residual seeding mixture with additional quantities of metal salts and reducing agents, to produce magnetic metal alloy particles of controlled size. Also of interest is the initiation of particle production without utilizing catalytic materials within the bath as reported in commonly assigned U.S. Pat. Nos. 3,726,664 and 3,859,130 in which a combination of amine borane and hypophosphite reducing agents is utilized to initiate production of metal alloy particles without a catalyst.
Control of magnetic characteristics of continuous alloy films deposited from electroless plating baths has been investigated and reported extensively. Perhaps the most complete report of the relationships between the coercivity of electrolessly plated cobalt films and bath parameters is to be found in U.S. Pat. No. 3,138,479. This reference teaches control of cobalt film coercivity by the combined control of pH (with NH.sub.4 OH) in the range of 7 to 9, agitation up to 350 RPM, sodium hypophosphite reducing agent concentration, temperature in the range of 140.degree. to 200.degree. F (60.degree. to 93.degree. C), and other parameters which are not germane to particle preparation, such as substrate preparation and film thickness. Of interest is the discovery reported in this reference that while each of these parameters had some combined effect on coercivity, pH is the most critical factor.
Neither cobalt-phosphorus film nor particles can be produced by hypophosphite reduction from an electroless bath which is not basic. Most work on the production of continuous cobalt-phosphorus films by electroless plating has been done in basic baths in which the pH is controlled with ammonium hydroxide. However, other bases have also been used in these bath compositions.
Ingredients which form complexes or chelates with cobalt cations are normally included in electroless plating baths whether they are intended to produce films or particles. Cobalt complexing and chelating agents include, for example, ammonia, the primary, secondary and tertiary amines, imines, mono- and di-carboxy groups, saturated unsubstituted short chain aliphatic dicarboxycylic anions and hydroxy groups. Control of coercivity in electrolessly plated cobalt films by controlling the concentrations or ratios of complexing and of chelating agents has been taught, for example, in U.S. Pat. Nos. 3,360,397; 3,423,214; and 3,446,657 and Tsu et al: IBM TECHNICAL DISCLOSURE BULLETIN, Volume 4, No. 8, page 52, January 1962.
Until recently, little has been reported on the control of magnetic properties of cobalt particles produced by decomposition of electroless baths. Results of studies concurrent with the present invention now indicate that the coercivity of magnetic Co-P particles is affected in many ways which would be predictable to one skilled in the art of electroless plating. It also indicates that coercivity is surprisingly unaffected by other parameters.
However, no relationship between the concentration of Pd cation catalyst and film or particle coercivity is known to have been reported.
Recently, methods of making finely divided magnetic cobalt-phosphorus alloy particles with selectively controlled high coercivity by chemical oxidation-reduction have been reported in commonly assigned U.S. Pat. No. 3,756,866. This is accomplished by dissolving a salt of cobalt in a bath rendered basic by a non-complexing source of hydroxyl ions and reducing the metal salt with hypophosphite anion while selectively controlling the temperatures of the bath, thereby precipitating cobalt-phosphorus particles having selected coercivity.
The present invention provides a highly effective alternative technique for producing novel finely divided amorphous magnetic cobalt-phosphorus particles 300 A and less having selectively controlled coercivity by controlled decomposition of a bath including a palladium cation catalyst having controlled concentrations. Additionally, the procedures and resulting Co-P particles taught by the present invention are unique, independent of its capacity for selectively controlling coercivity.
The production of magnetic recording media, for example, including particles having controlled coercivity can be critically important for data processing uses. This is so because such magnetic compositions may require that they be fabricated to possess a predetermined coercivity and thereby function predictably as recording media in the form of tapes, loops, drums, disks and the like. The coercivity desired may vary from one application to another. It is therefore seen that there is a great need for a technique for forming magnetic particles having predictable and reproducible controlled coercivity.
The use of catalytic material to assist in plating continuous metal films onto non-metallic surfaces by electroless techniques using hypophosphite anion reducing agents has been recognized since the inception of this technology by Brenner et al in U.S. Pat. Nos. 2,532,283 and 2,532,284. The use of palladium salts alone, or in conjunction with stannous chloride sensitizing solutions for plating continuous metal films onto non-metallic or non-catalytic surfaces, was taught in U.S. Pat. Nos. 2,690,402 and 2,702,253. From their first applications, electroless techniques were used to plate nominally magnetic films of cobalt, nickel and iron, but not in the form of recording media.
The application of electroless techniques to recording media production, including the step of applying a catalyst to the substrate, may first have been taught by U.S. Pat. Nos. 3,116,159 and 3,138,479. Specific requirements for the use of a catalyst, a review of the state of the art, and especially a catalyst solution of palladium chloride and hydrochloric acid, were set forth in U.S. Pat. No. 3,269,854. This latter reference teaches that cobalt film coercivity can be increased or controlled, independent of film thickness, by periodically interrupting the plating and re-exposing the plated surface to the catalytic solution. It does not suggest that the coercivity of plated cobalt film is a function of palladium cation concentration. Admittedly, various concentrations of palladium salts have been used in the prior art in various applications, but all such known uses had no apparent appreciation of any relationship between palladium cation concentration and plated cobalt coercivity.
The use of phosphoric acid or sulfuric acid with a palladium salt catalyst is taught by U.S. Pat. Nos. 3,423,226 and 3,437,507, respectively. It is believed that substitution of these and other acids for the hydrochloric acid of the present invention will provide equivalent results.